Question

Sodium azide, \(\mathrm{NaN}_{3}\), is the nitrogen gas-forming substance used in automobile air-bag systems. It is an ionic compound containing the azide ion, \(\mathrm{N}_{3}^{-}\). In this ion, the two nitrogen-to-nitrogen bond lengths are \(116 \mathrm{pm} .\) Describe the resonance hybrid Lewis structure of this ion.

Short answer

The azide ion, \(\mathrm{N}_{3}^{-}\), has a resonance hybrid structure in which the central nitrogen atom is interchangeably double-bonded to each of the other two nitrogen atoms and each nitrogen-to-nitrogen bond is a 1.5 bond due to the equal bond lengths of 116 pm.

Step-by-step solution

Sketch Lewis Structures

To start with, each Nitrogen atom has 5 valence electrons and we have 3 Nitrogen atoms, so we have 15 valence electrons. When we add the negative charge, that gives us one more electron, so we have a total of 16 electrons to distribute. There are several viable Lewis structures for the \(\mathrm{N}_{3}^{-}\) ion. One has a nitrogen atom in the center singly bonded (2 electrons each) to the two other nitrogen atoms and one lone pair on the center nitrogen atom, while the other nitrogen atoms each carry a negative charge and three lone pairs of electrons (6 electrons each). The other structure is similar, but with the negative charge and the additional bond on the other side.

Show the resonance

The actual structure of the azide ion is a resonance hybrid of these two Lewis structures. This is indicated by using a double-sided arrow between the two structures. The center nitrogen is doubly bonded to either of the other nitrogen atoms interchangeably, showing the non-localized nature of the pi electrons.

Represent the Resonance Hybrid

To represent the equivalent bond lengths in the resonance hybrid, we can draw a dashed line between the nitrogen atoms in the Lewis structure to show that each nitrogen-to-nitrogen bond is actually 1.5 bonds in the hybrid. This explains the equal bond lengths of 116 pm.

Key concepts

Lewis Structures

Lewis structures are vital for understanding molecules and ions in chemistry. They represent the arrangement of atoms, bonds, and electrons for a given molecule or ion. In the case of the azide ion \(\mathrm{N}_{3}^{-}\), we're interested in knowing how these nitrogen atoms share electrons to form bonds.
To create a Lewis structure, we count the total number of valence electrons. Each nitrogen atom contributes 5 valence electrons, and the negative charge adds one more, totaling 16 electrons. Distributing these electrons forms the basis of the Lewis structure.
A viable Lewis structure has one nitrogen atom in the center bonded to the two others. Each non-central nitrogen has a negative charge with three lone pairs. The central nitrogen has a lone pair too. More than one valid Lewis structure can exist, indicating resonance. This guides us to see the resonance among these structures.

Azide Ion

The azide ion \(\mathrm{N}_{3}^{-}\) is a fascinating ion due to its resonance properties. It plays a critical role in automobile airbag systems for generating nitrogen gas. The azide ion consists of three nitrogen atoms in a linear arrangement, and its electron configuration allows for different Lewis structures.
These structures depict the movement of \(\pi\)-electrons between the nitrogen atoms. The resonance in \(\mathrm{N}_{3}^{-}\) means there is no single, fixed structure for the ion. Instead, it is a hybrid of several structures where the electrons are delocalized across the three nitrogen atoms.
This delocalization leads to equal bond lengths in the azide ion, making it distinctively stable and uniform. This is why understanding resonance is essential when exploring compounds like \(\mathrm{N}_{3}^{-}\).

Nitrogen Bonds

Nitrogen bonds in the azide ion are an example of how resonance influences molecular structure. Unlike single or double bonds, the bonds in the \(\mathrm{N}_{3}^{-}\) ion are a mix of both – known as "bond order".
The resonance structures suggest each bond behaves as a 1.5 bond, showing equal contribution of single and double bond characteristics. This bond order of 1.5 results in equal nitrogen-nitrogen bond lengths of 116 pm.
Resonance allows electron pairs to be spread out, reducing repulsive interactions between the electrons. This creates stability in the azide ion. Understanding the nature of these bonds helps explain why bond length and strength aren't fixed but instead shared across the molecule.